12.7. Human Health

12.7.1. Diseases and Injuries

Impacts of climate and climate change on health can be direct or indirect.
Direct effects that are readily attributed to climate include heat stress and
the consequences of natural disasters. However, the resulting burden of disease
and injury may be less than that from indirect effects such as disrupted agriculture
and reduced food security. Positive and negative effects can be anticipated,
but there is insufficient evidence to state confidently what the balance will
be (see Chapter 9). We have not attempted to estimate
the overall economic costs of climate change impacts on health in the Australia-New
Zealand region because there is considerable debate about the derivation and
interpretation of monetary costs (see Chapter 19).

Guest et al. (1999) compared heat-related deaths in the five major Australian
cities in the period 1977-1990 with those expected under different climate
change scenarios (CSIRO, 1996a) for the year 2030. They estimate that greenhouse-induced
climate change would increase climate-related deaths in the summer by a small
amount, but this would be more than balanced by a reduction in climate-related
deaths in the winter. Overall, this resulted in a decrease of 8-12% in
climate-attributable mortality under the CSIRO "high" scenario compared
to a scenario with no climate changes (but expected population changes).

The only study in New Zealand to date of elevated temperatures and mortality
was conducted by Hales et al. (2000). Daily numbers of deaths in Christchurch
were compared with measures of weather and ambient particulate pollution from
June 1988 to October 1993. Above the third quartile (20.5°C) of summer maximum
temperatures, an increase of 1°C was associated with a 1.3% increase (95%
confidence interval, 0.4-2.3%) in all-cause mortality and a slightly greater
increase in mortality from respiratory conditions. There was no evidence of
interaction between the effects of temperature and particulate air pollution.
Greater than expected numbers of deaths also occurred during winter days of
low temperature, although this was not statistically significant. This suggests
that cold-related deaths will be less common in a warmer climate. However, the
mechanisms that explain excess winter mortality are not well understood.

The effects of solar ultraviolet (UV) radiation on skin cancer, skin aging,
and cataracts of the eye are particularly important in New Zealand and Australia,
which already have the highest skin cancer rates in the world (Marks et al.,
1989). The etiology of skin cancer is not fully understood, and factors other
than sun exposure undoubtedly are involved. However, UV-B is a key factor. Present
levels of UV radiation in this region are relatively high and have been increasing
in the past 20 years (McKenzie et al., 1999). It is expected with a high
degree of confidence that if UV flux at ground level increases at a faster rate
as a result of greenhouse-related cooling in the upper stratosphere and subsequent
slowdown in the breakdown of ozone-depleting substances, the incidence of melanomas
and other skin cancers will increase (Armstrong, 1994; Longstreth et al.
1998). Australians and New Zealanders of pale-skinned European descent will
be particularly vulnerable to these effects. This topic is discussed in more
detail in Chapter 9.

The numbers of notified cases of arbovirus infections (illnesses caused by
insect-borne viruses) have increased in Australia in recent years (Russell,
1998), and exotic insect species such as Aedes albopictus and Aedes
camptorhynchus that are competent vectors of viruses such as dengue and
Ross River virus have been detected at New Zealand borders (Hearnden et al.,
1999).

There is good evidence that the frequency of mosquito-borne infections in this
region is sensitive to short-term variations in climate. For example, outbreaks
of Ross River fever and Murray Valley encephalitis in southeast Australia tend
to follow heavy rainfall upstream in the Murray-Darling catchment (Nicholls,
1993; Maelzer et al., 1999). In other parts of Australia where the predominant
vector is the coastal mosquito A. camptorhynchus, variations in sea level
also contribute to outbreaks of illness from Ross River virus (Mackenzie et
al., 1994). No quantitative estimates have been made of the possible impact
of long-term climate change on rates of vector-borne infections. However, present
climate change scenarios suggest that parts of Australia and New Zealand will
experience conditions that are more favorable to breeding and development of
mosquitoes (Bryan et al. 1996). In these areas, warmer conditions will
tend to extend the range of reservoir hosts, decrease the extrinsic incubation
period of arboviruses, and encourage outdoor exposure of humans (Weinstein,
1997). Therefore, it is expected with a high degree of confidence that the potential
for insect-borne illness will increase. Whether this potential is translated
into actual occurrence of disease will depend on many other factors, including
border security, surveillance, vector eradication programs, and effectiveness
of primary health care.

Endemic malaria was present in North Queensland and the Northern Territory
until early in the 20th century (Ford, 1950; Black, 1972). Vectors to transmit
the disease still are present in that part of Australia, and climate change
will favor the spread of these mosquitoes southward (Bryan et al., 1996). The
disease-limiting factor at present is the effectiveness of local health services
that ensure that parasitemic individuals are treated and removed from contact
with mosquitoes. Therefore, climate change on its own is unlikely to cause the
disease to return to Australia, unless services are overwhelmed. In New Zealand
there currently are no mosquitoes that are capable of transmitting malaria;
even under global warming scenarios, the possibility of an exotic vector becoming
established is considered to be slight (Boyd and Weinstein, 1996).

Studies of the prevalence of asthma in New Zealand have shown an association
with average temperature (Hales et al., 1998). Electorates with lower mean temperatures
tend to have lower levels of asthma, after adjusting for confounding factors.
The reason is not clear but may be related to exposure to insect allergens.
If this were so, warming may tend to increase the frequency of asthma, but too
little is known about the causes of the disease to forecast the impact of climate
change on asthma in Australia and New Zealand.

Climate change may influence the levels of several outdoor air pollutants.
Ozone and other photochemical oxidants are a concern in several major Australian
cities and in Auckland, New Zealand (Woodward et al., 1995). In Brisbane, Australia,
current levels of ozone and particulates have been associated with increased
hospital admission rates (Petroeschevsky et al., 1999) and daily mortality in
persons ages 65 and over (Simpson et al., 1999). Outdoor particulate pollution
in the winter (largely generated by household fires) has been associated with
increased daily mortality in Christchurch, New Zealand (Hales et al., 2000).
Formation of photochemical smog is promoted in warmer conditions, although there
are many other climatic factorssuch as windspeed and cloud coverthat
are at least as important as temperature but more difficult to anticipate. A
rise in overnight minimum temperatures may reduce the use of fires and hence
emissions of particulates, but it is not known how this might affect pollution
and population exposures.

Toxic algal blooms may affect humans as a result of direct contact and indirectly
through consumption of contaminated fish and other seafood. At present this
is not a major public health threat in Australia or New Zealand, but it is an
economic issue (because of effects on livestock and shellfish). It could affect
very large numbers of people (Oshima et al., 1987; Sim and Wilson, 1997). No
work has been carried out in Australia or New Zealand relating the health effects
of algal blooms to climate. Elsewhere in the Pacific, it has been reported that
the incidence of fish poisoning (resulting from ingestion of fish contaminated
with ciguatoxins) is associated with ocean warming in some eastern islands,
but not elsewhere (Hales et al., 1999). It is uncertain whether these conditions
will become more common in Australia and New Zealand with projected climate
change.

Since 1800, deaths specifically ascribed to climatic hazards have averaged
about 50 yr-1 in Australia (Pittock et al., 1999), of which 40% are estimated
to be caused by heat waves and 20% each from tropical cyclones and floods. Although
this is not necessarily representative of present conditions because of changing
population, statistical accounting, and technologies, it is an order of magnitude
estimate. This suggests that if heat waves, floods, storm surges, and tropical
cyclones do become more intense, some commensurate increase in deaths and injuries
is possible. Whether this will occur will depend on the adequacy of hazard warnings
and prevention. Statistics from other developed countries with larger populations
indicate a recent trend toward increasing damages but decreasing death and injury
from climatic hazards. Thus, hazard mitigation is possible, although it must
more than outweigh increased exposure resulting from larger populations in hazardous
areas.